doc: document buffer.buffer property

Buffer objects expose the underlying `Uint8Array`'s `buffer` property
by default. This patch formally documents it.

PR-URL: nodejs#8332
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

doc/api/buffer.md

@@ -879,6 +879,19 @@ for (let i = 0; i < str.length ; i++) {
 console.log(buf.toString('ascii'));
 ```
 
+### buf.buffer
+
+The `buffer` property references the underlying `ArrayBuffer` object based on
+which this Buffer object is created.
+
+```js
+const arrayBuffer = new ArrayBuffer(16);
+const buffer = Buffer.from(arrayBuffer);
+
+console.log(buffer.buffer === arrayBuffer);
+// Prints: true
+```
+
 ### buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])
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